FIG. 18 shows the overall construction of a liquid-development electrophotographic apparatus according to the conventional art. A plurality of developing units are provided on and around an intermediate transfer body, one for each of yellow, magenta, cyan, and black colors. Each developing unit includes a photosensitive drum (photosensitive body) 11-14 and a developing roller.
The developing roller, which is biased to a predetermined voltage so as to generate an electric field between the developing roller and the photosensitive drum 11-14, causes toner to adhere to an exposed region on the photosensitive drum 11-14 in accordance with the electric field, and develops an electrostatic latent image on the photosensitive drum 11-14, thereby forming a visible image. Liquid toner is fed to the developing roller from a toner reservoir. The liquid toner is thereby applied onto the developing roller to a predetermined layer thickness.
The intermediate transfer body transfers to itself toner adhering to the photosensitive drums 11 to 14, in accordance with the electric field between the intermediate transfer body and the respective photosensitive drums 11-14. A total of four color images developed on the respective photosensitive drums 11 to 14 are thereby superposed on the intermediate transfer body in sequence while the intermediate transfer body makes a single rotation, thereby forming a multicolor image. From these four color images thus superposed on the intermediate transfer body, carrier liquid is removed by means of one of more carrier-removing units 15. The image formed from liquid toner on the intermediate transfer body contains carrier liquid; conventionally, all the carrier-removing units for removing this carrier oil are located downstream of the position where superposition of the plural color images is completed, and the carriers contained in the images of four colors, for example, are collected simultaneously. Notably, each carrier-removing unit 15 includes a roller or a belt which is equipped with collecting means for collecting oil, and bias applying means for applying a bias voltage to the carrier-removing unit 15 in such a direction as to press the respective color image against the intermediate transfer body, and which is brought into contact with the intermediate transfer body to remove excessive carrier from a toner layer formed on the intermediate transfer body. Then, at a contact zone where the four-color image meets a printing medium, the four-color image is transferred to the printing medium by use of a pressure roller, whereupon the transferred four-color image is fixed while passing through a fixing unit.
Carrier solvent in liquid development serves not only to prevent toner particles of around 1 μm from scattering, but also to assist in uniformly dispersing the toner particles in a charged state; particularly, in development and electrostatic transfer steps, the carrier solvent also behaves as a bridge so that toner particles can easily move under the influence of an electric field.
Carrier solvent in a liquid-development printer process is a component that is essential as the process progresses from preservation of toner, to conveyance of toner to form a toner layer, to development of toner, and finally to electrostatic transfer of toner. Subsequent to the fixing step in which the image is fixed to a paper medium, however, the carrier solvent is not necessary. In the case of liquid-developer toner employing a nonvolatile carrier solvent, the carrier solvent cannot be volatilized while the toner is heated to melt, and therefore the carrier solvent would retard build-up of adhesion of the melted toner to a paper medium, particularly during the fixing of toner and during the melting and transfer of toner. As a result, quality of the image transferred and fixed to the paper medium and adherence of the melted toner occasionally fail to be fully satisfactory.
Thus, in the case of the nonvolatile carrier solvent, carrier (nonvolatile liquid) on the intermediate transfer body, etc. must be removed and collected before the formed image is transferred and fixed to the medium (printing material). In the conventional apparatus illustrated in FIG. 18, the carrier-removing units 15 are disposed downstream of the position where superposition of the plural color images is completed, and the carriers contained in the images of four colors, for example, are collected simultaneously. In this structure, since the total amount of carrier existing on the intermediate transfer body would increase with the progress of superposition of the color images, transfer of the image from the individual photosensitive drum to the intermediate transfer body would become progressively difficult. Consequently setting up conditions for each developing unit would be necessary, and mistransfer of image or disturbance of the already formed image would be likely to occur. Further, simultaneous removal of carriers for the four color images is not performed to a sufficient degree, because cohesion of toner progresses insufficiently.
In addition, a method for removal/collection of carrier (nonvolatile liquid) on the intermediate transfer body, etc. is known. In this known method, a conductive or semiconductive collection roller to which a bias voltage is applied in such a direction as to press toner against a toner-bearing object is disposed in a confronting relation with the toner-bearing object, and carrier adhering to the collection roller is collected by means of a blade, for example. For this purpose, a highly efficient carrier removing technique is known in which the collection roller is rotated in such a manner that its surface moves in a direction opposite that of surface movement of the intermediate transfer body (hereinafter also called reverse-rotation). However, this reverse-rotation technique is acknowledged to exert considerable stress on the toner and hence be likely to impair the image.
In order to maintain image quality while collecting carrier by bringing a reverse-rotating roller, which is high in carrier collection efficiency, into contact with a toner-bearing body such as an intermediate transfer body, a shear force generated in the toner by the reverse-rotating roller must be smaller than the degree of cohesion of toner resulting from application of a bias voltage. Increasing the temperature/bias voltage is effective for enhancing cohesion for toner; meanwhile, effective measures to minimize the shear force occurring in the toner include reducing the contact force between the collection roller and the toner bearing body, smoothing the roller surface, and reducing the frictional resistance between the roller and the toner layer.
Further, the conventional technology encounters the following problems. In a liquid-development electrophotographic apparatus using liquid toner, when a roller or belt for collecting excessive carrier is brought into contact with an image-bearing body such as an intermediate transfer body while the roller or belt is stopped, contact marks (nip marks) are left on the roller or belt. When removal of carrier takes place in such a state, image quality is adversely affected. Consequently, preferably the roller or belt and the image-bearing body assume an out-of-contact posture while the apparatus is stopped or on standby.
However, when the roller or belt is brought into contact with the image-bearing body while a bias voltage is applied to the roller or belt, discharge would occur with respect to the image-bearing body before the roller or belt comes into contact with the image-bearing body, in view that the bias voltage for pressing toner against the image-bearing body is as high as more than 1 KV. Accordingly, the discharge may lead to malfunction of the apparatus and formation of pinholes on the surface of the roller or belt and the surface of the image-bearing body.
Furthermore, in the case of a liquid-development electrophotographic apparatus equipped with a plurality of carrier-removing units, when the individual carrier-removing units are simultaneously biased to high voltage, the liquid-development electrophotographic apparatus will occasionally malfunction as a result of noise arising at that time.